1. Field of the Invention
The present invention relates to a fail-safe apparatus for an image forming apparatus in which a CPU is used to control an image forming sequence.
2. Description of the Prior Art
In an image forming apparatus, such as a copying machine, a printer, or a facsimile system, in which a CPU (central processing unit) is used to control an image forming sequence, the CPU or a peripheral IC such as an input-output controller may be erroneously operated due to some abnormality, e.g., external noise. In this case, the overall drive, system of the image forming apparatus overruns, and the internal functions may be damaged In order to prevent this, a watchdog circuit is provided. The watchdog circuit always monitors the operation of the CPU by fetching predetermined pulse signals from the CPU and converting these pulses into a continuous signal. If no continuous signal appears for a predetermined time period, this circuit supplies a reset signal to the CPU or the peripheral IC so as to stop the sequence of the image forming apparatus, thus preventing damage.
Such a fail-safe apparatus is disclosed in detail in Japanese Patent Laid-Open (Kokai) No. 55-146457.
Generally, an error often occurs in an image forming sequence because of generation of noise (due to lightening) at a high-voltage portion of an image forming apparatus. In such a case, failure instantaneously occurs in a peripheral IC or in electronic circuits near a load. Then, even if the CPU is reset and the image forming sequence is stopped, a certain error condition may independently remain in each peripheral portion of the image forming apparatus. For this reason, such an apparatus is not as a fail-safe apparatus.
When the CPU is reset, the program in the CPU of the sequence is initialized and the operation is started in a new mode. Therefore, for example, if the sequence is for a feed operation to transfer paper this operation remains to be performed. That is, the transfer paper is in a jam state, and a jam indication lamp is turned on. However, it is not clear from the appearance of the state whether this indication light means a normal jam state or a jam state resulting from an operation of the watchdog circuit due to some abnormality. Therefore, when a serviceman repairs the apparatus, confirmation of the cause becomes difficult. As described above, the conventional prior art fail-safe apparatus is not perfect.